lnrpc: add betweenness centrality to GetNodeMetrics (new RPC call)

This commit extends the RPC interface with GetNodeMetrics will contain
all graph node metrics in the future. Currently only holds betweennes
centrality per node.

cmd/lncli/main.go

@@ -282,6 +282,7 @@ func main() {
 		closedChannelsCommand,
 		listPaymentsCommand,
 		describeGraphCommand,
+		getNodeMetricsCommand,
 		getChanInfoCommand,
 		getNodeInfoCommand,
 		queryRoutesCommand,